[5][6] Daniel Kastler is known in particular for his work with Rudolf Haag on the foundation of the algebraic approach to quantum field theory.

Their collaboration started at the famous Lille Conference in 1957, where both were present, and culminated in the Haag–Kastler axioms[7] for local observables of quantum field theories.

[8] In other collaborations, Kastler showed the importance of C*-algebras in the foundations of quantum statistical mechanics and in abelian asymptotic systems.

[9] In the 1980s he started working on Alain Connes' non-commutative geometry, especially studying the applications in elementary particle physics.

[10] In the same period Kastler, in collaboration with Raymond Stora, developed the geometrical setting for the BRST transformations for the quantization of gauge theories.